1. Field of the Invention
This invention relates to exercise equipment, and in particular to a force transmission mechanism for exercise machines to provide a resistance which varies with distance.
2. Description of the Prior Art
It is known to use various combinations of weights and pulleys in exercise machines, to provide a resistance for the person to work against. It is most common in such machines for the force transmission mechanism to have a constant mechanical advantage, whether 1:1 or some other ratio.
For example, one common machine is a so-called "hack lift", which consists of a generally vertical board with two parallel horizontal protrusions extending from each edge. The board is on a track and connected to a pulley system to which a weight is attached. The person faces away from the board and crouches so that his or her head is between the two protrusions and the shoulders contact the underside of each protrusion.
In a conventional hack lift, there is a constant mechanical advantage. That is, the resistance is constant throughout the lift. Thus a sudden exertion is required at the beginning of the lift, which is not desirable for several reasons. First of all, there is the potential for injury due to the large initial force required. Secondly, the constant mechanical advantage does not take into account the fact that the person is able to exert a much larger force as the legs approach being straight, as opposed to when the person is in a crouched position.
Thus in a hack lift, and in many other exercise machines, there is a need for a force transmission mechanism which includes a variable mechanical advantage, such that the resistance varies with position. In a hack lift, for example, it would be preferable for the resistance to be relatively low when the person is crouched, and to increase as the person's legs straighten.